Don't set the default port when the Host header exists

If there is a Host header, then we can expect the port to be part of it.
If the port is not there, then we should use the default for the scheme
as per RFC 2616, section 14.23.

Slim/Http/Uri.php

@@ -177,7 +177,7 @@ public static function createFromEnvironment(Environment $env)
         if ($env->has('HTTP_HOST')) {
             $host = $env->get('HTTP_HOST');
             // set a port default
-            $port = 80;
+            $port = null;
         } else {
             $host = $env->get('SERVER_NAME');
             // set a port default